The use of inkjet printing systems in offices and homes has grown dramatically in recent years. The growth can be attributed to drastic reductions in cost of inkjet printers and substantial improvements in print resolution and overall print quality. While the print quality has drastically improved, research and development efforts continue toward improving the permanence of inkjet images because this property still falls short of the permanence produced by other printing and photographic techniques. A continued demand in inkjet printing has resulted in the need to produce images of high quality, high permanence, and high durability, while maintaining a reasonable cost.
In inkjet printing, the inkjet image is formed on a print medium when a precise pattern of dots is ejected from a drop-generating device known as a print-head. The typical inkjet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet printhead array. The nozzles are typically 30 to 40 μm in diameter. The inkjet printhead array incorporates an array of firing chambers that receive liquid ink, which comprises colorants, pigments, and/or dyes dissolved or dispersed in a liquid vehicle, through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a firing resistor, located opposite the nozzle so ink can collect between the firing resistor and the nozzle. The printhead is held and protected by an outer packaging referred to as a print cartridge or an inkjet pen.
Upon energizing of a particular firing resistor, a droplet of ink is expelled through the nozzle toward the print medium to produce the image. Printed images that have good print quality are a function of both the ink composition and the print medium upon which the image is printed. Ideally, the ink composition would have a high degree of permanence, a fast drying time, a long shelf-life, and would provide the desired chroma with the desired fastness or permanence properties. Ideally, the print medium would have sufficient ink-absorbency, a high degree of permanence, and high dryability.
Various factors affect the permanence of the printed image, such as exposure to humidity, temperature, water, plasticizer, light, or atmospheric gases. Atmospheric gases that affect the permanence of the printed image include, but are not limited to, O2, O3, SO2, and oxides of nitrogen. The oxides of nitrogen include, but are not limited to, nitrous oxide, nitric oxide, nitrogen sesquioxide, nitrogen dioxide, dinitrogen tetroxide, dinitrogen pentoxide, and mixtures thereof. These atmospheric gases, which include oxidizing and reducing gases, fade or degrade the colors in printed images. The atmospheric gases are present in air and, therefore, printed images fade upon exposure to air.
The airfastness of the printed image is a function of both the ink composition and the print medium. The ability of the printed image to resist fading upon exposure to atmospheric gases is referred to as airfastness or resistance to airfade. The printed image is airfast if the printed image does not exhibit shifts in color or a decrease in color density upon exposure to atmospheric gases. It is known in the art that cyan colored inks are less airfast than magenta colored inks, which are less airfast than yellow colored inks. In addition, black inks are also known to fade upon exposure to atmospheric gases.
Colorants in the ink fade when exposed to atmospheric gases due to photodegradation mechanisms, which include oxidation or reduction of the colorants, electron ejection from the colorant, reaction with ground-state or excited singlet state oxygen, and electron or hydrogen atom abstraction to form radical intermediates. More specifically, the atmospheric gases generate free radicals that degrade the ink composition and/or the print media and generate more free radicals, which further accelerates the degradation process.
Airfade or gasfade in porous print media has only recently been identified as a significant problem and, therefore, few solutions to this problem have been proposed. One proposed solution is to add metal oxides to the print media, as discussed in Rolf Steiger, “Light Stability and Gas Fading on Nanoporous Ink-Jet Materials,” NIP17: International Conference on Digital Printing Technologies, p. 222–225. Other proposed solutions include forming a barrier layer over the printed image using lamination techniques or using low molecular weight hindered amine light stabilizers (“HALS”), antioxidants, and UV absorbers. While barrier layers are effective, their use is time consuming and cost intensive. The low molecular weight additives are also problematic because some of the additives that were used were sacrificial and, therefore, did not provide long term protection. Other additives, while being regenerative, were not effective due to their low volatility. In addition, only a few additives were tested in aqueous systems due to the limited solubility of the additives in water.
Airfade or gasfade has been a longstanding problem in the textile industry because textiles (such as clothing, carpets, etc.) are comprised of dyed fibers that are constantly exposed to atmospheric gases. On exposure to atmospheric gases, the dyed fibers fade or turn yellow. Low molecular weight additives, such as color stabilizers, have been added to the dyed fibers to improve their airfastness. These additives include antioxidants, HALS, UV absorbers, and free radical quenchers, and are typically oligomeric compounds that have lower volatility. Light stabilizers, such as HALS, react with free radicals and prevent these free radicals from generating additional free radicals.
For example, in U.S. Pat. No. 4,737,155 to Rollick et al, oxadiazine thiones and triazine thiones were added to dyes to improve their resistance to ozone. In U.S. Pat. No. 3,794,464 to Lofquist et al., polytertiary amines were added to dyed nylon fibers to improve their resistance to ozone. In U.S. Pat. No. 5,500,467 to Mahood, a phosphite and a hindered phenolic antioxidant or a HALS was added to polyolefin fibers to improve their resistance gasfade. In U.S. Pat. No. 5,596,033 to Horsey et al., a HALS was used to improve the gasfade of polypropylene fibers. In U.S. Pat. No. 3,988,292 to Moriga et al., a triazine derivative was used to improve the gasfade of polyurethanes and cellulose acetates. In U.S. Pat. No. 5,904,738 to Purcell, dyed textiles were treated with at least one polyalkylene imine to improve their resistance to gasfade.
In light of the problems associated with the airfastness of printed images, it would be advantageous to improve the airfastness of printed images using high molecular weight additives that have low volatility.